Ultrafast engineering of the magnetic Hamiltonian in frustrated magnets

Ultrafast light-matter coupling has emerged as a new mechanism to stabilize novel matter phases that are unstable in thermal equilibrium [1]. Of particular interest are quantum materials with coupled magnetic and structural instabilities. In these compounds, the targeted manipulation of the crystal lattice via strong THz pulses resonant with IR-active phonon modes can decouple the magnetic and structural degrees of freedom and alter their delicate energy balance [2]. Observation of this phenomenon in antiferromagnets, which should display faster dynamics and are of interest to spintronic applications [3], is yet to be demonstrated. We deploy ultrafast laser pulses to engineer the magnetic groundstate of an iridium-based frustrated quantum magnet [4] by resonantly pumping an IR active phonon mode that couples to a symmetry-breaking Jahn-Teller mode, and we engineer long-lasting changes to the magnetic Hamiltonian as demonstrated by our THz spectroscopy measurements. Our results open new pathways to controlling complex magnetic orders in strong spin-orbit coupled frustrated spin degrees of freedom.